1. Field of the Invention
The invention pertains to the field of automotive air conditioning compressor electromagnetic clutches. More particularly, the invention pertains to such a clutch with a reduced part count and large areas of sliding contact, which reduce stress and increase durability.
2. Description of Related Art
Almost all automotive air conditioning clutches are powered by a belt-driven pulley, a pulley that freely rotates about the drive shaft of the compressor, until an annular iron armature is pulled by an electromagnetic coil, against a friction disk of the pulley with enough force to cause the two to stick frictionally together. The armature, in turn, is physically supported on a central hub of the compressor drive shaft by an armature support mechanism that holds the armature coaxially to and spaced away from the pulley friction disk, close enough to be pulled into and against it when the clutch is actuated. The armature support mechanism must be radially and circumferentially rigid enough to successfully transfer drive torque from the co-rotating pulley and armature to the shaft. It also must allow the armature to move freely toward the pulley, when the clutch is turned on, and pull it away from the pulley as the clutch turns off. Ideally, the armature should be supported on the shaft hub by a mechanism that not only transfers torque and provides axial flexibility, but which also provides a measure of torsional flexibility and resilience, enough to reduce the shaft torque peaks from the pumping action of the compressor pistons. This is called a torque cushion design.
The typical prior art armature support mechanism consists simply of a rigid drive plate welded to the compressor drive shaft hub, and three or more simple leaf springs riveted at one end to the drive plate, and at the other end to the armature. The leaf springs lie in a plane parallel to the drive plate, and are fairly rigid in that plane, capable of transferring torque between the armature and drive plate and ultimately to the compressor drive shaft. However, the springs can flex easily in the axial direction, in cantilever fashion, since they are thin in the axial direction. This is an advantage, because it permits a low magnetic force to successfully pull the armature to the pulley friction disk. Examples may be seen in several issued patents, for example, U.S. Pat. No. 5,046,594 to Kakinuma, where armature 26 is joined to a hub mounted drive plate 28 by three thin leaf springs 27.
This is a very common design. Three leaf springs with two rivets each make 9 parts. Including the shaft hub, drive plate, and armature makes a total of 12 parts for this type of clutch, excluding the pulley.
Other designs use an annular rubber ring to perform both the torque cushion function and the flexible spring function. These designs are very stiff in the axial direction, a disadvantage, because a very high magnetic force is needed to pull the armature to the pulley friction disk. The rubber torque cushion outer diameter typically is bonded to an outer ring wall, which in turn is fastened to the armature with three or more rivets. The inner diameter of the rubber torque cushion typically is bonded to an inner ring wall, which in turn is riveted or welded to the shaft-mounted hub. The hub, torque cushion inner and outer bonding rings with three rivets each, and armature totals 10 separate parts.
The invention provides an armature with a multiple array of elongated piercings. The shaft-mounted hub has a similar array of elongated lugs, which fit closely into the armature piercings. A single spring anchored to the hub body pushes the armature away from the pulley friction disk, when the clutch is turned off. This spring may have a very low axial force, an advantage cited above. All rivets, drive plate, weld, multiple flat leaf springs and ring walls have been eliminated, resulting in an assembly of only three parts: the shaft hub, armature and spring. The small number of parts is a great advantage, substantially reducing manufacturing costs and increasing the reliability of the mechanism.
In the preferred embodiment, an iron armature, coaxial and opposed to a pulley friction disk, has fifteen (15) elongated radial piercings. The radial length of the armature piercings, combined with the thickness of the armature itself, provides a large bearing area to support the closely fitting and opposed sides of the shaft hub lugs. When the clutch is turned on, the tangential forces originating from the pulley belt are carried by the total edge area of the fifteen (15) armature piercings to the opposed hub drive lug surfaces. This large total support area for the tangential driving forces results in a low compressive stress between the armature edges and the sides of the hub lugs. Any rubber torque cushion pads interposed between the opposed faces of the armature piercings and hub lugs is then subject to only low compressive stress, which increases the durable life of the rubber dramatically.
In an alternative embodiment, no rubber torque cushion is used, substantially reducing the cost of the assembly. In this embodiment, the large metal to metal contact area between the armature piercings and hub lugs has a very low wear rate, due to the low compressive stress over this area. This has the advantage of prolonging the useful life of the clutch.
In both the preferred embodiment and the alternate embodiment, multiple concentric areas support the concentricity of the armature to the hub. First, the outer diameter of the hub itself closely fits within the internal diameter of the armature. The thickness of the armature provides a cylindrical support area between the hub outer diameter and armature inner diameter. Second, the areas of the inboard ends of the hub lugs are radially opposed to the thickness of the armature at the inner end of the armature piercings. Third, the outboard ends of the hub lugs are radially opposed to the thickness of the armature at the outer ends of the armature piercings. The sum of the three concentric areas of radial support provides a very large total circumferential contact area between the armature and shaft hub. This large circumferential area supports any radial forces arising from operation of the clutch, and results in very low radial compressive stress. This reduces the wear rate between the opposed circumferential surfaces to a minimum, prolonging the useful life of the clutch.
In both the preferred and alternate embodiment, one end of a single compression spring in direct contact with the armature pushes it from the pulley friction disk, when the clutch is turned off. The other end of the compression spring is closely fitted into and anchored by a groove in the hub outer diameter. The compression spring optionally is made from coiled wire, a Belleville washer, or a solid elastomeric material, such as rubber, or any other similarly suitable material.